Strengthening layer for use in composites to be formed by means of vacuum technique

ABSTRACT

The invention relates to a strengthening layer for composites to be formed by means of vacuum technique, which layer substantially consists of a strengthening material and for a smaller part of substantially round transport threads for guiding the resin therealong, which threads are substantially form-retaining in cross-section and lie substantially in the direction of the resin transport.

The present invention relates to a strengthening layer for composites tobe formed by means of vacuum technique, as well as to an assembly ofstrengthening layers and composites containing the strengthening layer.

Vacuum techniques are used to make composites in a closed mould system.A vacuum is produced in the mould by means of a small opening on anouter end of the mould, while the resin is supplied at the other end viaa small opening. The resin will fill the mould due to the underpressurein the mould.

Quite simple moulds are usually applied in such techniques. In somecases such moulds consist only of a lower mould with a cover foil. Whena vacuum is produced in the mould there results an external pressure onthe mould of about 1 bar. When the mould is partially flexible, such aswhen a cover foil is applied as upper mould, this pressure is alsoexerted on the strengthening layer present in mould. Since the mould asit were collapses and the strengthening layer is herein also compressed,the resin transport is in many cases made more difficult or evenprevented.

Because the vacuum technique can be used with relatively low mouldcosts, it is an advantageous method. To nevertheless enable the use ofthis vacuum technique with these simple moulds use is currently made ofso-called “bleeders”. These are intermediate layers inserted between thestrengthening layers so as to still allow resin transport. Such“bleeders” are for instance continuous glass mats, which cannot becompressed by the external pressure on the mould and its content. Such“bleeders” are however generally a disruptive element in the end productsince they cause unnecessary thickness, weight, resin and materialconsumption. Furthermore, they leave behind a resin-rich location in theend product, thereby creating the danger of air inclusion.

The present invention therefore has for its object to provide astrengthening layer which is particularly suitable for formingcomposites by means of vacuum technique.

This is achieved with the invention by a strengthening layer whichsubstantially consists of a strengthening material and for a smallerpart of transport threads of substantially round and substantiallyform-retaining cross-section for guiding the resin therealong, whichthreads lie substantially in the direction of the resin transport.Because of the form-retaining nature of the transport threads they arenot flattened by the external pressure but retain their form. It hasbeen found that sufficient openings thereby remain along the threads toenable resin transport. The transporting speed of the resin can beinfluenced by the choice of the thread thickness and by the number ofthreads.

The transport threads can lie in the same plane as the rest of thestrengthening material. In that case they preferably replace a part ofthis material, whereby no extra weight is added. The invention ishowever also effective when the transport threads are situated betweenthe different layers of strengthening material.

Although the problem of the strengthening material being compressed canoccur with different types of strengthening material, it is a particularproblem in the case of strengthening material which takes at leastpartly the form of endless threads lying substantially paralleladjacently of each other, or multifilaments. Multifilaments are bundlesof separate threads which are not mutually attached to each other withina bundle and are also not intertwined. They therefore spread into a flatsurface in the case of external pressure.

The form-retention of the transport threads according to the inventioncan be achieved in different ways. It is thus possible to apply two ormore twined single threads or a torsional single thread. In addition,form-retention can also be achieved by means of a substantially rigidcoating applied to the whole or partial surface of the thread. Such acoating again ensures that the threads cannot be compressed. Although itis evident that a coating over the whole surface gives the best result,it is however also possible, in order to save material, to suffice witha coating in parts at intermediate distances such that sufficientrigidity is provided for the threads to prevent compression thereofunder a pressure of about 1 bar. As coating can for instance be used aglue or other finishes known in the art of textile processing.

When the transport threads are monofilament threads, they already formone whole and for this reason cannot be flattened. A similar result canbe achieved when the transport threads are provided with a sheath. Sucha sheath can for instance be a knit or a braiding. The form-retention ofthe transport threads can likewise further be ensured in that they formpart of a structure of threads which are mutually connected by a bindingsuch that the round form of the transport threads cannot be distorted,or hardly so. Examples of such structures are for instance gauzes orwebs manufactured according to Rachel technique.

Both the strengthening materials and the transport threads according tothe invention can be formed from the usual strengthening materialsapplied for the reinforcement of plastics, such as glass, carbon,kevlar, flax, other vegetable or synthetic fibres or combinationsthereof. The material applied for the transport threads can be the sameas the rest of the strengthening material or different. Glass is mostlyused as strengthening material and as material for the transportthreads.

A strengthening layer according to the invention will in practicegenerally be applied in an assembly of a plurality of layers. Such anassembly comprises at, least one strengthening layer according to theinvention and can in addition comprise for instance rovings and mats. Itis however recommended to employ the transport threads distributeduniformly over the different strengthening layers, since the mosthomogenous possible resin distribution is then obtained.

The invention finally extends to composites which consist of at leastone layer or assembly according to the invention embedded in resin.

By using one or more strengthening layers according to the invention inthe manufacture of composites using the vacuum technique, a compositewithout intermediate layer (bleeder) can now be made with a great numberof advantages. Firstly, a resin-rich layer is prevented from forming inthe middle of the laminate. There is the danger of air inclusion takingplace in such a layer. In addition, by choosing the same material forthe transport threads as for the rest of the layer it is possible toprevent the end product containing laminate-foreign substances. Sincethe transport threads according to the invention preferably replace apart of the rest of the strengthening material, the composite will notneed a greater thickness than is actually desirable. Since according tothe invention no extra layer is needed and the threads are preferablyused to replace a part of the strengthening material, no unnecessaryconsumption of resin is required. An additional bleeder, which has nofunction in the final laminate, moreover increases the weight of thelaminate. This is also prevented by the transport threads according tothe invention. Further prevented when the thickness is the same is thatless strengthening can be inserted. Since a bleeder itself already has arelatively great thickness, less real material providing strengtheningcan be used. The bleeder is formed from criss-cross fibres which provideno or hardly any strengthening in one direction.

In this application “composite” is understood to mean a material inwhich the supporting function is taken over almost entirely by thereinforcement material (strengthening material). The resin herein has asealing character and functions for the purpose of cohesion and stresstransfer.

“Vacuum technique” is understood to mean the suction of resin by meansof vacuum through a closed mould having therein one or morestrengthening layers for the purpose of forming a composite.

A “strengthening layer” is a quantity of strengthening materialdisplaying cohesion. The term “strengthening material” is used for thecomponents of which a strengthening layer consists, such asmultifilaments, glass rovings and so on.

The invention will be further elucidated with reference to the examplefollowing hereinbelow.

EXAMPLE Comparison of Resin Transport in Different Types ofStrengthening Material

1. Test Arrangement

The elongate strengthening layer for testing was placed between twosheets of foil functioning as mould. The foil sheets were sealed allround in order to enable creation of a vacuum in the mould. A vacuum wasapplied at the one end of the mould, while resin was fed into the mouldat the other end. A measuring rule was positioned in longitudinaldirection of the mould to enable determining of the distance covered bythe resin per unit of time.

2. Strengthening Materials As first comparison material an assembly ofstrengthening layers was manufactured from six layers of unidirectionalwebs, wherein in longitudinal direction each layer had a weight of 1125g/m² glass rovings and in transverse direction a weight of 75 g/m² glassrovings. A glass mat of 50 g/m² was attached hereto. Such an assemblycannot be formed without any problem into a composite by means of vacuumtechniques. A continuous glass mat of 450 g was therefore placed as“bleeder” between the third and fourth layer of unidirectional webs. Thethickness of this continuous mat was roughly 0.8 mm. The thickness ofeach layer of unidirectional web was 0.9 mm.

As second comparison material the above described assembly ofstrengthening layers was used without the bleeder.

In the material according to the invention 20 g/m² glass roving in theabove mentioned web of 1250 g/m² was replaced by twined glass yarn inthe direction in which the resin transport has to be directed. The webis not changed appreciably by this replacement.

3. Result

By means of the comparison material with the stated (continuous mat)bleeder the resin can be transported over about 35 cm in 15 minutes withthe vacuum technique. Without the continuous glass mat as bleeder thetransport is found to amount to only 10 cm in 15 minutes. The resintransport in the material according to the invention amounts to 35 cm in15 minutes.

It can be seen from the foregoing that with much less material and insimple manner an at least equally good result can be obtained accordingto the invention as when a bleeder is applied.

What is claimed is:
 1. A strengthening layer for composites comprising a resin to be formed by means of a vacuum technique, comprising: a strengthening material; and a plurality of transport threads of substantially round and substantially form-retaining cross-section for guiding the resin therealong wherein said threads lie substantially in the direction of the resin transport.
 2. The strengthening layer as claimed in claim 1 wherein the transport threads lie in the same plane as the rest of the strengthening material.
 3. The strengthening layer as claimed in claim 1 wherein the transport threads lie against one or both sides of the strengthening layer.
 4. The strengthening layer as claimed in claim 1 wherein the strengthening material takes at least partly the form of endless threads lying substantially parallel adjacently of each other.
 5. The strengthening layer as claimed in claim 1 wherein the form-retention of the transport threads is achieved in that said threads comprise at least two twined single threads.
 6. The strengthening layer as claimed in claim 1 wherein the form-retention of the transport threads is achieved in that said threads comprise torsional single threads.
 7. The strengthening layer as claimed in claim 1 wherein the form-retention of the transport threads is achieved in that said threads further comprise a coating applied to at least a partial surface of the thread.
 8. The strengthening layer as claimed in claim 7 wherein the coating is a glue.
 9. The strengthening layer as claimed in claim 1 wherein the form-retention of the transport threads is achieved in that said threads are monofilament threads.
 10. The strengthening layer as claimed in claim 1 wherein the form-retention of the transport threads is achieved in that said threads are provided with a sheath.
 11. The strengthening layer as claimed in claim 10 wherein the sheath comprises a knit.
 12. The strengthening layer as claimed in claim 10 wherein the sheath comprises a braiding.
 13. The strengthening layer as claimed in claim 1 wherein the form-retention of the transport threads is achieved in that said threads form part of a structure of threads which are mutually connected by a binding such that the round form of the transport threads is substantially form-retaining during the vacuum technique.
 14. The strengthening layer as claimed in claim 13 wherein the form-retention of the transport threads is achieved in that said threads form part of a gauze.
 15. The strengthening layer as claimed in claim 13 wherein the form-retention of the transport threads is achieved in that said threads form part of a web manufactured according to the Rachel technique.
 16. The strengthening layer as claimed in claim 1 wherein the transport threads are formed from at least one of glass, carbon, kevlar, flax, other vegetable, synthetic fibres and combinations thereof.
 17. The strengthening layer as claimed in claim 1 wherein the strengthening material is formed from at least one of glass, kevlar, flax, other vegetable, synthetic fibres and combinations thereof.
 18. The strengthening layer as claimed in claim 1 wherein the transport threads are manufactured from at least one of the same materials as the strengthening material.
 19. The strengthening layer as claimed in claim 1 wherein said layer is embedded in resin.
 20. The strengthening layer as claimed in claim 4 wherein the strengthening material at least partly takes the form of multifilaments.
 21. The strengthening layer as claimed in claim 4 wherein the transport threads lie in the same plane as the rest of the strengthening material.
 22. The strengthening layer as claimed in claim 4 wherein the transport threads lie against one or both sides of the strengthening layer. 